Data storage in magnetic storage media systems, e.g., a linear tape storage system, has arrived at the point where progress in density requires implementation of track following servo systems to allow narrower data tracks. For example, tape products without servo generally operate with far fewer tracks per cm than systems which utilize servo technology.
The recording and reading of data in tracks on magnetic storage media requires precise positioning of magnetic read/write heads. The read/write heads must be quickly moved to and centered over particular tracks as recording and reading of data is performed. The heads can record and read data as relative movement occurs between the heads and the magnetic storage media in a transducing direction, e.g., the movement of tape in a tape cartridge across a read/write head. The heads are moved from track to track across the width of the tape in a translating direction which is perpendicular to the transducing direction.
Generally, magnetic storage media devices that read and record data on magnetic media use servo control systems to properly position the data read/write heads in the translating direction, e.g., across the width of the tape, perpendicular to the transducing direction, e.g., a direction coincident with the length of the tape. The servo control system generally provides a position signal from one or more servo read heads that read cross width position control information recorded in one or more servo tracks on the magnetic storage media.
Conventional servo systems generally fall into a category referred to as boundary systems in which at least one servo track or servo track pattern extending along the length of the magnetic media, e.g., tape, is laterally divided into two or more regions, separated by linear boundaries. The distinct regions have different properties which can be detected by the servo read head. For example, the regions may be recorded at different frequencies or phases, or they may contain bursts occurring at distinct times. Generally, for example, the servo head elements straddle the boundary between the regions, and the ratio of the amplitude of the response of the servo head to each region provides the position signal upon which the track following servo operates.
Such boundary type servos and tape systems are particularly susceptible to errors in the position signal. For example, to provide sufficient lateral dynamic range, servo read head elements tend to be approximately as wide as a full servo track width. Servo head instabilities, head wear, localized debris on the head or tape, and media defects all contribute to temporary or long-term shifts in the spatial response of the servo head to the recorded servo pattern in the servo track.
To address the problems associated with boundary type servos in which such systems are particularly susceptible to errors in the position signal as described above, time-based servo systems for magnetic storage media have been described. U.S. Pat. No. 5,689,384 to Albrecht, et al., entitled “Timing Based Servo System For Magnetic Tape Systems,” issued 18 Nov. 1997, describes a track following servo control system for use with magnetic media tape systems which derives head position information from one or more specially patterned servo tracks. The servo patterns are comprised of magnetic transitions recorded at more than one azimuthal orientations in a servo track, such that the timing of the servo position signal pulses derived from reading the servo pattern are decoded to provide a position signal used by the servo system to position data heads over desired data tracks of the storage media.
For example, the servo pattern described in U.S. Pat. No. 5,689,384 may comprise straight transitions essentially perpendicular to the length of the track alternating with azimuthally inclined or sloped transitions. That is, the azimuthally included or sloped transitions may extend across the width of a track at an angle to the head transducing direction. The relative timing of transitions read by the servo read head varies linearly depending on the head position with respect to the center of the servo track. A position signal is generated by determining the ratio of two timing intervals. In particular, the ratio can be determined by normalizing the variable time interval between dissimilar transitions with the interval measured between like transitions. A read head that is narrow with respect to the width of the servo track pattern and the data track width can be used.
Such a time-based servo has numerous advantages over boundary servo systems. For example, such a time-based servo generally addresses the disadvantages associated with the use of wide servo read heads. However, such time-based servo systems, depending upon the type of time-based servo pattern recorded in the servo track and the method of recording such a time-based servo pattern, may be sensitive to tape speed variations at the time the servo pattern is recorded in the servo track. Because of the need to control tape speed variations when recording time-based servo patterns, specialized equipment which includes special speed control features is generally necessary to perform such time-based servo recording. As such, the time based servo recording process becomes complex and expensive. Further, during playback of data recorded on such magnetic storage media, the time-based servo, depending upon the type of servo pattern recorded in the servo track, may be sensitive to transverse tracking variations, e.g., across the width of the servo track.